I know this matter crops but here goes with my own specific récent experience.
I changed form landline to mobile phone + internet connection recently. So I lost my old landline number and gained a new mobile number as a ‘virtual landline.’ It has the mobile number system (eg starts with 06) and a handset that hangs of my new mobile router but nevertheless functions like a landline.
I’m now getting lots of cold calls on that line with nobody on the end of the line. By contrast I just bought a new SIM for my real mobile phone - same service provider (SFR); a prepaid (i.e. not on a plan) service but get no cold calls on that.
So what’s going on?